cxl: Reduce scope for the variable “mm” in cxllib_get_PE_attributes()

A local variable was used only within an if branch.
Thus move the definition for the variable “mm” into the corresponding
code block.

This issue was detected by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/5cee2b25-71e0-15aa-fba6-12211b8308aa@web.de